Handle sharing from other apps (Android)

[borisyankov]

Register app so other apps can send it content (text, urls, photos).

[jainkuniya]

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[zulipbot]

Hello @vishwesh3, you have been unassigned from this issue because you have not updated this issue or any referenced pull requests for over 10 days.

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[jainkuniya]

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will resume work on #674